156 THE EVOLUTION OF SEX. 



SUMMARY. 



1. Reproduction is but more or less discontinuous growth. 



2. Sexual reproduction normally implies {a) special reproductive cells, 

 distinct from the body ; (/') the dimorphism of these cells ; {c) their 

 physiological dependence, — the ovum being unproductive without the sper- 

 matozoon, and z'ice versa. 



3. The discoveries of Camerarius, Amici, Kolreuter, Sprengel, and 

 others, laid the foundations of our knowledge of sexual reproduction in 

 plants. 



4. The history of research on fertilisation in animals well illustrates the 

 gradually increasing precision of scientific inquiry. 



5. The conjugation processes seen in Protozoa are of much importance 

 in suggesting the origin of differentiated fertilisation. 



6. The origin of fertilisation may be traced through the following 

 grades : — {a) plasniodial union, (/') multiple conjugation, (r) ordinary con- 

 jugation, {d) union of dimorphic cells, {e) fertilisation of ovum by sperma- 

 tozoon. 



7. Both in plants and animals hybridisation is often successful, but the 

 offspring frequently tend to be sterile. This, however, must not be 

 exaggerated. 



LITERATURE. 



See the already noted v/orks of Balfour, Van Beneden, Carnoy, Geddes, 

 Haddon, Hensen, Hertwig, jNI'Kendrick, Sachs, and Vines. 



P^or recent papers see Boveri, Th., Zellen Studien ; Jenaische Zeitschrift 

 fur Naturwissenschaften, 1SS7-88 ; Zoological Reco/d, from 1886; and 

 Journal of Royal Microscopical Society. 



